1. Field of the Invention
This application relates to systems employing content addressable memory (CAM) arrays in which a memory is searched on the basis of the contents of the memory data rather than by the data location in the memory.
2. Description of the Prior Art
The conventional or von Neumann-type computers, and especially those using principles of addressing computational variables by the indices of their locations, have frequently been regarded as unsatisfactory in large problems. Many attempts have been made to replace the conventional random-access memories by structures from which the operands could be called out by their symbolic names or data contents, and possibly many operands at a time. The high-level algorithmic and problem-oriented languages might take significant advantage of such a feature. There exist at least two reasons for which development in this direction has not been particularly rapid. One of them is the much higher price of content-addressable memories compared to the random-access memories. Another reason is that the problem of encoding the variables symbolically already has effective software solutions which do not require special memories. This, however, does not bring about the other feature which would be very desirable in large problems, namely, retrieving of a great number of variables from the memory simultaneously.
While associations were originally considered for the description of interrelations or cross references between pieces of information only, it has later turned out that searching of data by its partial content can effectively be utilized in the manipulation of arithmetic algorithms. Such content addressing can be made in a highly parallel fashion, i.e., simultaneously over a great number of data elements, usually at a rather low level, referring to transformations that occur in the binary representations. The problem of parallel computation has also another direction where content-addressability is used to control higher-level algorithmic functions in parallel.
The following definitions may be employed in connection with the subject matter of the present invention.
Content-addressable memory: a storage device that stores data in a number of cells. The cells can be accessed or loaded on the basis of their contents.
Content-addressable processor: a content-addressable memory in which more sophisticated data transformations can be performed on the contents of a number of cells selected according to the contents, or a computer or computer system that uses such memory as an essential component for storage or processing, respectively.
Accessing data on the basis of their content always involves some comparison of an external search argument with part or all of the information stored in all cells. Whether this is done by software, mechanical scanning or parallel electronic circuits, is immaterial in principle; however, a "genuine" content-addressable memory performs all comparisons in parallel. Another fact to emphasize is that comparison by equality match between the search argument and the stored item is not the only mode used. If the stored data have numerical values, the purpose of content-addressable search may be to locate all cells the contents of which satisfy certain magnitude relations with respect to the search arguments; for instance, being greater than or less than the given limit, or between two specified limits. Content-addressable searching is sometimes performed without reference to an external search argument, for instance, when locating the maximum or minimum in a set of stored numbers.
Hardware content-addressable memories have been used as special parts in computing systems, in certain organizational solutions whereby the CAMs can effectively perform fast buffering, checking, and bookkeeping operations needed in the moving and rearrangement of data. In this way the CAM devices can effectively contribute to the speed of usual operations by making the operands more directly available to the processing circuits.
In a masked search in a CAM array, only a subset of the bits of a search-argument word is compared with respective bits of all memory words. Masking in the CAM usually means masking out, or disablement of certain bits. Those stored words which agree in the specified (unmasked) bit positions with the search argument are then read out.